Autoranging method and system for testing the speeds of a camera shutter

Abstract

A camera shutter tester that automatically displays a standard shutter speed nearest to the actual speed of the shutter being tested and displays a ratio of that speed to the standard speed. A photodetector is placed in the focal plane of the camera that faces a light source and the photodetector's output is threshold detected and a logic circuit generates an initialization pulse. A voltage controlled oscillator dependent upon the photodetector's output feeds a countdown shift register that is in turn fed by a range shift register which also feeds range information to a ratio counter. The initialization pulse activates the range shift register, the countdown register, and the ratio counter. The frequency of the voltage controlled oscillator is divided by the output of the countdown shift register and the new frequency is fed to the ratio counter which in turn delivers a range change pulse to the range shift register. The output of the range shift register and the ratio counter are fed to displays that are latched by the threshold detector.

What is claimed is: 
     
       1. A method for measuring shutter speed in the camera comprising: a. positioning the shutter adjacent to a light source;   b. activating the shutter causing a pulse of light to pass therethrough;   c. sensing the intensity and duration of the light pulse;   d. automatically displaying a standard shutter speed which most accurately represents the effective speed of the tested shutter; and   e. displaying an error ratio relating the effective speed of the shutter tested to the standard speed display.   
     
     
       2. A method for measuring shutter speed according to claim 1 which further comprises displaying of the standard shutter speed and error ratio by means of digital readout. 
     
     
       3. A method for measuring shutter speed in a camera comprising: a. positioning the camera adjacent to a light source;   b. activating the camera shutter so that a pulse of light passes therethrough;   c. sensing the intensity and duration of the light pulse;   d. generating a train of clock pulses for which the instantaneous frequency is proportional to the intensity of the light pulse;   e. counting said clock pulses; and   f. displaying the shutter speed as a function of the counted pulses.   
     
     
       4. A method for measuring shutter speed according to claim 3 wherein the shutter speed is displayed digitally. 
     
     
       5. A method for measuring shutter speed according to claim 3 wherein the light pulse sensing is at the focal plane of the camera. 
     
     
       6. A method for measuring the shutter speed in a camera comprising: a. positioning the camera adjacent to a light source;   b. activating the camera shutter so that a pulse of light passes therethrough;   c. sensing the intensity and duration of a light pulse at a point in the focal plane of the camera;   d. generating a train of clock pulses for which the instantaneous frequency thereof is proportional to the instantaneous intensity of the light pulse;   e. counting the train of clock pulses;   f. automatically displaying a standard shutter speed which most accurately represents the effective speed of the tested shutter; and   g. displaying an error ratio relating the measured effective shutter speed to the standard shutter speed display.   
     
     
       7. A method for measuring shutter speed according to claim 6 wherein the displaying of the standard shutter speed and the error ratio is performed digitally. 
     
     
       8. A method for measuring shutter speed according to claim 7 which further comprises varying the sensitivity of the sensing to accommodate different intensities of light sources. 
     
     
       9. A method of measuring shutter speed according to claim 1 which further comprises automatically resetting prior to a subsequent measurement. 
     
     
       10. A method for measuring shutter speed in a camera according to claim 9 which further comprises preventing erroneous readings when a timing period exceeds the unambiguous range of the instrument. 
     
     
       11. A method of measuring shutter speed according to claim 10 wherein the standard shutter speeds are that of the American National Standards Institute. 
     
     
       12. A system for testing the shutter of a camera comprising: a. a photodetector positioned at the focal plane of the camera;   b. a variable threshold voltage comparator connected to the photodetector;   c. a first oscillator;   d. means for generating an initialization pulse fed by the variable threshold voltage comparator and the first oscillator;   e. a countdown shift register fed by the initialization pulse generating means;   f. a voltage controlled oscillator fed by the photodetector;   g. means for dividing the frequency of the voltage controlled oscillator by a variable divisor, the variable divisor being generated from the output of the countdown shift register;   h. a range shift register activated by the initialization pulse generating means;   i. logic means for reloading the countdown shift register fed by the range shift register;   j. a ratio counter fed by the frequency dividing means and activated by the initialization pulse generating means;   k. logic means for reloading the ratio counter fed by the range shift register;   l. means for generating a range change pulse to activate the range shift register, the range change pulse generating means being fed by the ratio counter and the range shift register; and   m. a ratio display fed by the ratio counter and latched by the variable threshold voltage comparator.   
     
     
       13. A system for testing the shutter of a camera according to claim 12 which further comprises a range display fed by the range shift register and latched by the variable threshold voltage comparator. 
     
     
       14. A system for testing the shutter of a camera according to claim 13 which further comprises a variable gain amplifier interposed between the photodetector and both the voltage controlled oscillator and the variable threshold voltage comparator.